gui: RPC client, download table model, and a live main window

First vertical slice of velox-gui, built entirely against tools/mockd
(no daemon dependency):

- rpc/: RpcConnection runs a QLocalSocket on a worker thread with
  newline-delimited JSON-RPC framing, drives the session.hello /
  session.subscribe handshake, and reconnects with exponential backoff
  (250 ms -> 8 s). RpcClient is the main-thread face: marshals calls onto
  the worker, delivers replies as main-thread callbacks, re-emits server
  notifications as typed Qt signals, and issues the one-shot download.list
  on reaching Connected.
- models/DownloadTableModel: QAbstractTableModel over TaskSummary. A
  progress batch is a row patch with a narrow dataChanged over the value
  columns only; beginResetModel() is reserved for the initial load and a
  reconnect resync.
- widgets/ProgressDelegate: in-cell progress bar for the Status column.
- mainwindow/MainWindow: the table, a status-bar connection dot, an
  offline banner instead of a modal, dialog-free pause/resume/stop
  actions, and QSettings column/geometry persistence.
- gui/CMakeLists.txt links velox::proto (never velox::core, ADR 0009) and
  self-guards on the veloxproto target so main keeps configuring if it is
  ever absent again.
- i18n from the first commit: every string via tr(), plus an Arabic .ts
  stub for the RTL check.
- tests/: headless QTest for the model — proves the progress patch is a
  narrow dataChanged and never resets the model.

Verified end-to-end against `mockd --tasks 300`: handshake, initial list,
live progress batches applied to the model, and a clean
Reconnecting -> Connected recovery when mockd is bounced mid-run.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_016Ne28kx4VreeBWZv82Nksd

gui/src/main.cpp

@@ -0,0 +1,54 @@
+// velox-gui entry point. Lane GUI.
+
+#include <unistd.h>
+
+#include <QApplication>
+#include <QLibraryInfo>
+#include <QLocale>
+#include <QString>
+#include <QTranslator>
+
+#include "mainwindow/MainWindow.hpp"
+#include "rpc/Protocol.hpp"
+#include "rpc/RpcClient.hpp"
+
+namespace {
+
+QString defaultSocketPath() {
+    const QString override = qEnvironmentVariable("VELOX_SOCK");
+    if (!override.isEmpty()) {
+        return override;
+    }
+    QString runtimeDir = qEnvironmentVariable("XDG_RUNTIME_DIR");
+    if (runtimeDir.isEmpty()) {
+        runtimeDir = QStringLiteral("/run/user/%1").arg(getuid());
+    }
+    return runtimeDir + QStringLiteral("/velox/velox.sock");
+}
+
+}  // namespace
+
+int main(int argc, char **argv) {
+    QApplication app(argc, argv);
+    QApplication::setApplicationName(QStringLiteral("velox-gui"));
+    QApplication::setOrganizationName(QStringLiteral("velox"));
+    QApplication::setApplicationVersion(QStringLiteral("0.1.0"));
+
+    qRegisterMetaType<velox::gui::rpc::ConnectionState>();
+    qRegisterMetaType<velox::gui::rpc::RpcReply>();
+
+    // i18n from the first commit: load the bundled catalogue for the system locale.
+    // The Arabic stub (i18n/velox_ar.ts) exists to prove the RTL layout survives.
+    QTranslator translator;
+    const QString locale = QLocale::system().name();
+    if (translator.load(QStringLiteral("velox_") + locale, QStringLiteral(":/i18n"))) {
+        app.installTranslator(&translator);
+    }
+
+    velox::gui::rpc::RpcClient client(defaultSocketPath());
+    velox::gui::MainWindow window(&client);
+    window.show();
+    client.start();
+
+    return QApplication::exec();
+}